Conventional process for fabricating a thin film light emitting diode (TF-LED) roughly contains two phases. The first phase is to grow epi layers on a growth substrate and thus forming the epi wafer. The growth substrate can either be made of sapphire or silicon carbide. The number of epi layer can be designed according to the need. The second phase is to bond the epi wafer to a support substrate (such as a sub-mount or a packaging substrate), to remove the growth substrate, and to perform further semiconductor processes such as etching, photolithographing, development and phosphor coating. During the fabricating process of TF-LED, it's difficult to measure the photoelectric properties, such as the characteristics of current-voltage or spectrum of the epi wafer. Accordingly, said photoelectric properties of TF-LED are inspected and measured after the completion of two-phase process of TF-LED.
In the above process, particularly in the second phase of making a TF-LED, the semiconductor process is performed onto the entire epi-layer bonded with the support substrate. The photoelectric properties interim are hardly to be inspected, leading to a poor yield rate of TF-LED to 50% or even worse. More specifically, only 50% or fewer chips, though bonded with the support substrate, could meet the predetermined photoelectric properties. This means that all the chips, whether they meet the pre-determined photoelectric properties or not, have to undertake the subsequent fabrication process. For the chips that fail to meet the required photoelectric properties, the bonding with the carrier substrate appears to be unnecessary and a waste. Noted that LEDs to meet the required bins standard is often the challenge to most of the manufacturers when competing among one another. Therefore, promoting the yield rate of LEDs and cost-down is always the important issue to each LED maker.
Light emitting diodes (LEDs) have high brightness, low volume, low power consumption and long operating lifespan and as such, are used in a variety of display products. The luminescent principle of LEDs is as follows. A voltage is applied to a diode to drive an electron and a hole combination. The combination releases light from the diode. A conventional thin GaN LED product is manufactured by bonding an epi wafer to a carrier substrate (such as a sub-mount substrate or a package substrate)
FIG. 1 is a sectional view illustrating a conventional semiproduct LED epi wafer mounted on a carrier substrate 212. Referring to FIG. 1, the LED structure 210 includes a carrier substrate 212 and an epi wafer 214 mounted through a connecting layer 216 of the carrier substrate 212. The epi wafer 214 includes a growth substrate 218, and semiconductor layers 220 sequentially stacked on the growth substrate 218.
There are several problems associated with the conventional semiproduct LED package described above. First, the thickness of the connecting layer 216 between the carrier substrate 212 and the epi wafer 214 must be within a specific range, since a connecting layer 216 that is too thin, results in poor electric connection and adhesion to the carrier substrate 212. Further, in order for bonding, heat and pressure are applied to bond the epi wafer 214 to the carrier substrate 212. At this time, if the bonding material of a connecting layer 216 that is too thick, the heat applied would make the connecting layer 216 to protrude laterally and may cause a short circuit 223 of the semiconductor layers 220, as shown in FIG. 2. Such a short circuit 223, may cause the LED chip to lose its functional abilities. Second, due to the high temperature required for bonding, the semiprodcut LED epi wafer 210 suffers from residual stress after cooling. In the subsequent process of laser lift-off (LLO), dimensional restrictions of laser beams will make the semiconductor layers 220 to crack may occur after removal of the whole growth substrate 218. Third, since a support substrate (not shown) is preferably waived, the epi wafer is directly bonded to preformed circuits or traces in carrier substrate 212 for electric connection. The growth substrate 218 with semiconductor layers 220 blocks the sight of these traces or circuit. Therefore, misalignment occurs between the epi wafer 214 and the carrier substrate 212.
In other aspect, conventional method for manufacturing a thin film light emitting diode (LED) would bond a complete wafer, which will be diced to form a plurality of LEDs afterwards, on a substrate by heating the substrate and the wafer thereon. If the wafer has a non-uniform surface, the wafer is prone to fracture on the process of bonding. Besides, the conventional bonding temperature is about 400° C. and is regarded as a high temperature that would influence the quality of the wafer. Therefore, after the substrate and the wafer are bonded together and cooled down to a room temperature, a non-uniform stress distribution would occur to the wafer, weakening the structure of the wafer, and further affecting the sequential manufacturing process of LEDs.
At present, a conventional thin film LED normally has the problem of limited light emitting efficiency due to the deposition of metal electrode. For example, the metal electrode is directly disposed on the light emitting surface of the thin film LED. Thus, for a 12-mil LED, one-third light emitting area is usually lost due to the metal electrode. And for a 40-mil LED, one-ninth light emitting area is wasted. Besides, the electric current on the connection portion between the LED and the metal electrode is normally the highest, and is likely to cause energy loss.
The quality of light emitting device, such as the light-emitting diode (LED), is also very dependent on the luminance uniformity. After the LED dies have been fabricated in accomplishment, many LED dies are attached on a carrier or substrate for packaging. In packaging process, the florescent material or generally called wavelength conversion material is coated over the LED dies to produce the light, such as white light.
In the case of massive production, the process of die attachment process usually cannot be ideally controlled. As a result, the LED dies are usually not aligned to the ideal position. For example, some LED dies may be twisted by a certain angle and the location may be shifted from a designed location. Further, a mask layer for filling the florescent material on the LED die may also have fabrication errors and/or be in miss-alignment, for example. As a result, the florescent material would be non-uniformly coated over the LED dies, causing difference between the LED dies resulting in different luminosity in use. In addition, each LED die itself may further have non-uniform luminance in different illuminating angles. When the LED dies are composed into a light source, the luminance of the light source would be non-uniform as well. How to improve the uniformity of luminance of LED units in fabrication is an issue to be further developed.
Therefore, it is desirable to devise a novel light emitting diode package that improves upon the aforementioned problems.